Syria's White Helmets release harrowing footage of 'double tap' air strike in Idlib

The Syrian Civil Defence, better-known as the White Helmets, have released harrowing footage showing a "double tap" air strike targeting its civil rescue workers in the rebel-held province of Idlib.

The White Helmets published the video filmed by one of its members on Sunday, showing rescuers being bombarded while putting out a fire at the scene of a suspected Russian air raid in the town of Khan Sheikhoun.

"The exact moment White Helmets teams were targeted with cluster bombs and artillery shells while they were rescuing the wounded and extinguishing a fire following a Russian raid," the group said.

It added that two of its rescuers - Anas Diab and Mohammed al-Sahh - were injured in the bombing, which took place on Friday.

The attack appears to be a "double tap", aimed at killing emergency workers and victims at the scenes of initial bombings.

The White Helmets said the attack was carried out by Syrian and Russian warplanes, which have been pounding the rebel-held province in recent days.

The attack comes as Syrian troops - backed by Russia and Iran - have massed around the northwestern province of Idlib ahead of an expected onslaught against the largest rebel-held zone left in the country.

The UN's new humanitarian chief warned on Monday that a large-scale military operation against of Idlib could create "the worst humanitarian catastrophe" of this century.

Since 2015, Idlib has been home to a complex array of anti-regime forces: secular rebels, Islamists, Syrian jihadis, with ties to al-Qaeda, and their foreign counterparts.

The province is home to some three million people - around half of them displaced from other parts of the country, according to the United Nations.

A major military operation in Idlib is expected to pose a humanitarian nightmare because there is no nearby opposition territory left in Syria where people could be evacuated to.